This invention relates to solid polymer electrolyte rechargeable electrochemical storage cells. More particularly, this invention relates to organic redox shuttle additives for solid polymer electrolyte rechargeable electrochemical storage cells to provide overcharge protection to the cell.
Electrochemical storage batteries of all types are susceptible to damage due to overcharging or overdischarging. Overcharging of an electrochemical storage cell in a battery may be defined as charging beyond a cell's capacity, or at a rate greater than the cell's ability to accept such charge. The damage to the cell which may occur from such overcharging may include degradation of the electrodes, the current collectors, the electrolyte, and the separators between the electrodes. In addition, internal shorting and gas evolution which may result from such overcharging can result in unstable and even dangerous conditions.
Protection against overcharging of a single cell, or a battery comprising a small stack of series-connected cells, may be achieved through direct monitoring (potentiometric, galvanometric, thermal, etc.), control of charging rates, and state of charge. However, for a large (typically bipolar) stack of cells of the magnitude required, for example, in batteries for use in electric vehicles, these methods are impractical due to their complexity, weight requirements, and expense. Underutilization of capacity or addition of immobile electroactive chemicals to one or more of the electrodes may provide some protection (at a considerable cost), but such techniques are ineffective against significant deterioration of capacity within a single cell, which is generally cumulative, and which may lead to a short or an open circuit.
For cells utilizing liquid electrolytes, a "redox shuttle" has been proposed as an approach to solving the problem of overcharging. This approach employs an electrolyte additive which is inactive under normal conditions, but which oxidizes at the positive electrode when the cell potential exceeds the desired voltage, i.e., when the cell is in an overcharge state. The oxidized form of the shuttle additive diffuses through the cell to the negative electrode where it is reduced to its original (unoxidized) state and then the reduced form of the redox shuttle species diffuses through the cell back to the positive electrode to continue the redox cycle. The net effect is an internal shunt which prevents damage to the cell by imposing a limit on cell potential.
However, the requirements for such a "redox shuttle" additive are stringent. The shuttle material, in both its oxidized and reduced forms, must be nonreactive with all cell components. The onset potential for oxidation of the shuttle material must be slightly above that of the desired maximum cell potential to prevent self-discharge during storage, and to allow for some overpotential during charging. Furthermore, the shuttle additive must be present at a sufficient concentration and have a high enough diffusion coefficient (in both its oxidized and reduced forms) to be capable of providing a shuttle current at least as great as the current rate at which the cell is being charged. Additional desirable properties for the additive include a low equivalent weight, low volatility, low toxicity, and low cost.
With respect to the concentration and diffusion coefficient of the additive, such requirements are summarized for a parallel electrode configuration by the following expression: EQU I.sub.s =(nFADC/d)exp{(E-E.sup.0)nF/RT}/[1+exp{(E-E.sup.0)nF/RT}](1)
where:
I.sub.s is the current carried by the redox shuttle additive;
n is the number of electrons transferred to or from the shuttle at the electrodes;
F is the Faraday constant;
A is the electrode area;
D is an effective diffusion coefficient taking into account the diffusion properties of both the oxidized and reduced forms of the shuttle;
C is the total concentration of the diffusion species;
d is the distance between the electrodes;
E is the cell potential; and
E.sup.0 is the potential of the redox shuttle couple.
For E&gt;&gt;E.sup.0, the limiting shuttle current becomes: EQU I.sub.s =nFADC/d. (2)
Various shuttle materials have been suggested for use in rechargeable lithium cells utilizing liquid electrolytes. For example, Narayanan et al., in "Analysis of Redox Additive-Based Overcharge Protection for Rechargeable Lithium Batteries", published in the Journal of the Electrochemical Society, Vol. 138, No. 8 (1991) pp. 2224-2229, suggests the use of 1,1'-dimethylferrocene as a shuttle material in a liquid electrolyte comprising LiAsF.sub.6 and 2-methyltetrahydrofuran, while Abraham et al. in "Overcharge Protection of Secondary Lithium Batteries", published in the Proceedings of the 33rd International Power Sources Symposium in 1988, suggested the addition of lithium halides (LiBr and LiI) to the same liquid electrolyte.
The use of organic shuttle additives in liquid electrolytes of rechargeable lithium cells has also been proposed. Halpert et al., in "Status of the Development of Rechargeable Lithium Cells'", published in the Journal of Power Sources, 47 (1994) at pp. 287-294, disclosed results from the use of tetramethylphenylene diamine as an additive in a liquid electrolyte for a lithium-titanium disulfide rechargeable cell. The use of metallocenes, such as ferrocene, as additives to a liquid electrolyte for overcharge protection has also been suggested by Golovin et al., in "Applications of Metallocenes in Rechargeable Lithium Batteries for Overcharge Protection", Journal of the Electrochemical Society, Vol. 139, No. 1 (1992), at pp. 5-10.
However, the application of such technology to solid polymer electrolytes such as, for example, solid polyethylene oxide filled with a compound such as lithium trifluoromethanesulfonimide, which acts both as an ionically conducting electrolyte and as an electronically insulating separator, has not been reported, probably because of the many problems which would be associated with the use of an overcharge-limiting shuttle material in solid polymer electrolytes. For example, such solid polymer electrolyte/separators must be used at elevated temperatures (temperatures of at least about 60.degree. C.) where additive volatility is greater, and their higher viscosity will result in slower diffusion rates, especially for large molecules